1. Technical Field of the Invention
The present invention relates in general to the field of analog-to-digital converters (ADCs), and in particular, by way of example but not limitation, to digital calibration of ADCs involving a correction table that is adapted to compensate for wide-band calibration. The calibration may optionally be accomplished adaptively with dynamic estimation of reference signals that have unknown parameters.
2. Description of Related Art
The natural world operates in an analog domain, but information signals (voice, data, etc.) may frequently be processed, transmitted, or otherwise manipulated more efficiently in the digital domain. The conversion from the analog domain to the digital domain is accomplished with ADCs. An ADC receives as input an analog signal and produces as output a digital signal. However, some information present in the analog signal is necessarily lost during the conversion process even if an ADC is operating in an ideal manner. Unfortunately, real-world ADCs do not operate in an ideal manner. Consequently, the digital output of a real-world ADC does not track the analog input even as accurately an ideal ADC.
It is therefore beneficial to make and/or tune real-world ADCs to approximate ideal ADCs. Techniques have been developed to calibrate real-world ADCs so as to modify their performance to emulate ideal ADCs as closely as possible. For example, ADCs are traditionally calibrated using high precision digital voltmeters to characterize the errors that result from digitizing static or slowly varying analog reference voltages. The outcome from this static testing forms the basis for a hardware or software implemented (e.g., table look-up) calibration scheme. Another method of conventional ADC calibration is the use of a sinusoidal reference signal. The reference is sampled, and estimations of the ideal sample values are calculated. These estimations are calculated using a minimum squared error criterion that requires knowledge of the frequency of the calibration signal. The errors (i.e., the difference between the estimated values and the actual sampled values output by the ADC being calibrated) are then used to build a correction table. The correction table may subsequently be used to modify sampled values of actual (e.g., non-calibration, functional, etc.) analog input signals.
Efficient calibration schemes require that the reference signal be dynamically estimated on a sample-by-sample basis during the ADC calibration period(s). No method previously existed for dynamic estimation of a reference signal (e.g., a calibration signal) with one or more unknown parameters (e.g., frequency, phase, etc.) during an ADC calibration. The pre-existing calibration procedures relied on accurate and costly signal generators and/or precise and expensive measuring components.
However, a parent application (U.S. Ser. No. 09/196,811, now U.S. Pat. No. 6,127,955) of this patent application addressed these deficiencies of pre-existing calibration procedures by dynamically estimating a reference signal having one or more unknown parameters. Nevertheless, the invention of the parent application was primarily directed, with respect to frequency estimation, to the problem of calibrating ADCs that operate in a narrow frequency band. Since the linearity errors in general are frequency dependent, correction tables in accordance with the invention of the parent application are primarily useful for frequencies near the calibration frequency. The invention of the parent application does not therefore address the problem of wide-band calibration of ADCs. Consequently, implementations in accordance with the invention of the parent application do not optimally calibrate ADCs that are to be operated in a broad frequency band.
Furthermore, in existing approaches that do attempt ADC calibration by employing a sinusoidal reference signal that alternates between/among a number of fixed frequencies (instead of remaining at only a single fixed frequency), any correction table built using the reconstructed reference signal (whether from a minimum-mean-squared-error criterion technique or otherwise) is still deficient with respect to wide-band calibration. No frequency information is utilized when accessing the correction table, during either creation or functional use. Consequently, existing ADC calibration approaches entail either fully narrow-band calibrations with correction tables that are only calibrated for frequencies close to the single calibration frequency or partly wide-band calibrations with correction tables that are only calibrated for the average error over the number of fixed frequencies of the sinusoidal reference signal. Because ADC errors are frequency dependent, no existing approach fully calibrates an ADC over a wide frequency band.
The deficiencies of the prior art are overcome by the methods, systems, and arrangements of the present invention. For example, as heretofore unrecognized, it would be beneficial if the accessing of (e.g., the storing to and/or the retrieving from) correction tables compensated for a wide frequency band. In fact, it would be beneficial if such wide-band compensation were achieved by accessing an ADC calibration correction table using all or part of one or more past and/or future (e.g., previously buffered xe2x80x9cfuturexe2x80x9d values of) ADC output(s) as well as the current ADC output.
Methods, systems, and arrangements enable true wide-band calibration of analog-to-digital conversion using correction table indexing. The frequency of an analog input signal that corresponds to a digital output sample is accounted for, at least partially, when accessing a correction table during both calibration and functional use thereof. For example, with respect to certain embodiment(s), in addition to at least a portion of a current sample, at least portion(s) of one or more previous and/or subsequent sample(s) may be used to build (e.g., by bit concatenation) an index for addressing a correction table memory. In effect, compensation may be achieved for ADC errors that are frequency dependent. This correction table indexing approach may advantageously be employed along with, for example, a scheme that estimates one or more parameters of the analog input signal in the digital domain on a sample-by-sample basis in order to reconstruct the analog input signal in the digital domain. When the corresponding parameter estimation and signal reconstruction filter(s) are converged, a wide-band corrective data structure may be updated using, e.g., a concatenated index.
The above-described and other features of the present invention are explained in detail hereinafter with reference to the illustrative examples shown in the accompanying drawings. Those skilled in the art will appreciate that the described embodiments are provided for purposes of illustration and understanding and that numerous equivalent embodiments are contemplated herein.